refactor(pypi): return a list from parse_requirements (#2931)

The modeling of the data structures returned by the `parse_requirements`
function was not optimal and this was because historically there was
more logic in the `extension.bzl` and more things were decided there.

With the recent refactors it is possible to have a harder to misuse data
structure from the `parse_requirements`. For each `package` we will
return a struct which will have a `srcs` field that will contain easy to
consume values.

With this in place we can do the fix that is outlined in the referenced
issue.

Work towards #2648